1. Field of the Invention
This invention relates to a pathlength type microwave modulator using a waveguide.
2. Description of the Prior Art
Firstly, a microwave modulator according to the prior art will be explained with reference to FIG. 6. In FIG. 6, the microwave modulator includes a main waveguide GM which has a circulator C arranged in a path of the main waveguide GM and a short-circuiting waveguide GB which is branched at the circulator C. A diode D is loaded across H-planes of the branched waveguide GB at a position .lambda..sub.g /4 (where .lambda..sub.g is a guide wavelength) distant from a short-terminal TS of the short-circuiting waveguide GB towards the circulator C. In the above arrangement, when a pulse signal is applied to the diode D as a modulation signal, a carrier inputted from an input port of the main waveguide GM (left end as viewed in FIG. 6) is subjected to 0-phase and .pi.-phase modulation synchronized with ON-OFF switching operation of the diode D, so that a PSK signal is produced at an output port of the main waveguide GM (right end as viewed in FIG. 6) as a modulated output signal. The above arrangement is well known in the art.
However, the prior art shown in FIG. 6 involves the following problems.
(1) By loading the diode D, an insertion loss (about 2 dB in a millimetric wave, for example) arises in the OFF period of the diode D, and the loss becomes twice (about 4 dB) when the wave returns back reflected from the short-terminal TS. This results in a drawback arising in that amplitude modulation is imparted to the PSK-modulated output signal of the main waveguide GM because of the ON-OFF operation of the diode D.
(2) The short-circuiting effect of the diode D, when it is ON, is not so perfect so that there may be a wave reflected back from the short-terminal TS. This results in a drawback arising in that the amplitude and phase of the PSK-modulated output signal varies or in other words, a switching ratio of the output signals according to the 0-phase and .pi.-phase output levels is deteriorated.
(3) There is another drawback in that the length of the short-circuiting waveguide GB is fixed, and it is difficult to adjust the frequency of a modulated signal on the output end and to make fine tuning.